Even if Severa was wrong, the game isn't subtle about Cordelia being in love with someone. Multiple entire support conversations are about it without specifying who until the S rank. Even if you married her to Robin, Kellam, or Donnel, where it isn't mentioned, as soon as Severa states her suspicions it's clear who the person was that she had a crush on.
And even if you discount all that, it's still awful trying to pair her up and getting constantly reminded that she loves someone else. Even if she's come to love the person you pair her up with more, it's just off putting that they'd even bring up Chrom. There's no reason to for most people, really. Someone like Frederick or Stahl, or are either fiercely loyal or a bit more wishy washy it makes sense for, but others like Lon'qu just make no sense.